Recent estimations indicate that, on the average, each aircraft in the U.S. commercial aircraft fleet receives a lightning discharge about once each year. The aircraft generally experiences the lightning discharge when flying through a heavily charged portion of a cloud. In such cases, the discharge generally originates at the aircraft and extends outwardly from the aircraft. While the discharge is occurring, it generally moves from the nose of the aircraft and onto a plurality of skin panel portions of the aircraft as it moves through the charged region. The discharge may also attach to wing tips and/or edges of wing control surfaces (e.g., ailerons) during the discharge. The discharge then generally leaves the aircraft structure through the empennage. Since commercial aircraft contain relatively large amounts of fuel, and also generally include sensitive electronic equipment such as navigational computers and communications equipment that may be damaged by a lightning discharge, commercial aircraft are required to comply with a comprehensive set of certification procedures in order to verify that the aircraft is sufficiently protected from the potentially damaging effects of a lightning discharge.
Today's aircraft are being designed and built with greater percentages of composite material. Although composites are lighter and may have better mechanical and fatigue properties than traditional aluminum, they are less electrically conductive, and have poor electromagnetic shielding, causing poor current dissipation when lightning strikes the aircraft. Without an adequate conductive path, arcing and hot spots can occur that may char, delaminate and/or penetrate an aircraft's composite skin, and may reduce the load-bearing characteristics of the aircraft structure. For example, lightning may attach to a fastener and then flow though the fastener into a sub-structure below the surface (or skin) of the aircraft. The low electrical conducting capacity of composite materials may increase the likelihood that sub-structures may be adversely affected by the lightning strike, particularly when lightning attaches to a fastener, if additional safety techniques are not implemented.
Therefore, techniques to improve an airframe's tolerance to lightning strikes, particularly when lightning attaches to aircraft fasteners, have utility and may improve the economy and/or safety of air transit.